Finfish aquaculture has become prominent ever since natural finfish stocks have begun to be depleted due to over-fishing. Data from Marine Biologist Boris Worm, at Dalhousie University in Halifax, Canada, indicates that “by the middle of this century, fishermen will have nothing left to catch.” [Source: “Ocean's of Nothing”, page 101, Time Magazine, Nov. 13, 2006]. As a result, aquaculture is now “the fastest growing agri-food industry in the world” and is expected to outpace commercial fish landing by 2030 [Source: “Fin-fish Farming in British Columbia (2005)” BC Chamber of Commerce]. Lagging behind in this rapid growth industry is environmentally sustainable technology, which is now only beginning to develop. Open-net ocean-based pens are most common today, an example being the system featured in U.S. Pat. No. 7,284,501 which involves a containment pen composed of removable net panels. Systems such as these are often criticized by environmentalists for fostering disease and sea lice among finfish populations. There are also problems with escapes and with the passage of other pathogens and antibiotics into the marine ecosystem.
Sea lice are crustacean parasites, which attach themselves onto farmed and wild salmon. While sea lice do not directly kill fish, as parasites they draw down the fish's resources and decrease fitness, rendering them less viable. Sea lice also create open lesions on fish, which can lead to infection and can interfere with the fish's salt-water balance. The harm from sea lice is generally not fatal to adult fish, but increases mortality among juvenile salmon. Full-grown sea lice are a significant size and weight in comparison to smolt-size salmon, and can impede the smolt's ability to swim. Sea lice bred inadvertently in large numbers in farm salmon are viewed by environmental groups and some governments as a threat to wild salmon populations.
There is also the problem of escapes. Salmon farming on the west coast of North America is currently conducted in open net-pens in the marine environment. Farmed salmon can escape from their holding facilities and survive in the wild. The concern is that escaped salmon may reproduce and compete for spawning space and food with wild salmon.
Because net pens are open to ocean waters, pollution from salmon farms is discharged directly into the ocean. Waste from fish farms may include feces, uneaten food pellets, dead fish, antifoulants used on nets to reduce marine growth, disinfectants, and trace amounts of other materials used at fish farm sites.
These and other environmental concerns have led some state governments or provincial governments, such as those of Alaska and British Columbia to restrict or place moratoriums on salmon farming, particularly with open-net pens. These increasing environmental regulations and increasing consumer awareness have led to the development of a few land based aquaculture systems.
Land-based aquaculture systems are generally small in scale, but are expensive to build and operate due to the use of costly real estate, the cost of energy to pump water and the collateral costs and impact regarding existing sewage and waste management. One example of such a system is found in U.S. Pat. No. 5,014,647 which has a number of small fish tanks or silos connected to a pump by piping through which water is withdrawn from the tops of the silos by gravity and then delivered by a pump to the bottoms of the silos. Systems such as U.S. Pat. No. 5,014,647 are small scale and land based and they tend to have high unit production costs. The size of land based systems is also limited, due to the fact that large heavy tanks may collapse under their own weight. They also lack the buoyant material construction and the cost advantages of the marine based aquaculture systems and it is difficult to provide an adequate amount of exercise for the fish in such restricted environments.
With marine based closed-tank systems, the material used is an important consideration. Most marine based systems in commercial operation today that do not use open net-pen systems, use synthetic or metallic materials such as heavy gauge plastic or aluminum in their container construction. This results in problems with structural integrity, corrosion, resulting in short life and costly maintenance of such systems. Additionally, the inside surface of the tank must be composed of a material which should be USDA approved as being food-safe.
In terms of construction, there are three main types of fish tanks: raceway, D-ended, and rounded. These tanks are described below:
Raceway is a general term given to a straight sided artificial channel in which fish are held. Generally these have a high water turnover rate, occurring in less than one hour. The advantages of raceways are that they can be easily built in series, with the water flowing from one to the other and that they are easy to empty of fish by using a simple crowding screen. To be self cleaning, raceways must be operated at high flow rates and/or high stocking densities, where the movements of the fish keep feces and uneaten feed from settling. Disadvantages of raceways relate primarily to poor mixing, and include the gradual deterioration of the water quality along the length of the raceway (whereas round tanks tend to be more even). Difficulty in efficient distribution of additional oxygen throughout the raceway can prove to be another disadvantage. Raceways are usually built with a width to depth ratio of between 2:1 and 4:1, with the length limited either by the amount of fish that can be held in a single holding unit or the deterioration of water quality. The even nature of raceways means that they are somewhat flexible, allowing screens to be placed anywhere along the length of the raceway, thus dividing a single unit into 2 or more smaller units. Modifications to the designs include rounded bottoms to concentrate solids for ease of cleaning by vacuuming—especially where small fish are involved. Aeration along the length of the raceway may also be included, which serves to maintain more even oxygen concentrations along the length of the raceway, and also concentrates settled solids into specific areas to make cleaning easier. Other modifications include the addition of barriers in the raceway to create a swirling motion in the water where waste concentrates. Raceways are advantageous in that they can be constructed with basic building materials such as bricks, blocks or poured concrete and require little specialized labour.
D-ended raceways tanks are preferable for land based applications as they are very economical in terms of space. These tanks can be constructed from most materials, including fiberglass and concrete. They enable a lower tank turnover time, without compromising velocity rates and self cleaning abilities. Inlet pipes and aeration/oxygenation devices are positioned to create the desired water velocity rate. D-ended tanks are useful in situations where space and make up water are limited.
Round or cylindrical tanks have the advantage of a naturally self cleaning action. Circular motion of water within the tank induces a secondary toroidal flow, which sweeps wastes toward the center drain. Due to this property, they are often used in hatcheries, where due to high feed rates, solids loadings (waste feed and feces) can be very high and also in re-circulation systems, to remove the unwanted solids as soon as possible, before they break down. Round tanks can be constructed of almost any material, the most common being fiberglass (for tanks 8 m diameter and under), steel (lined or unlined) and concrete or concrete block. Other materials can be used as long as it is strong enough to hold the water without distortion and is non-corrosive, non-abrasive and non-toxic. Round tanks generally have a slope on the bottom towards the center outlet to increase solids removal efficiency. Other qualities of round tanks include: a good mixing of the water, resulting in easy oxygenation; and less contact of the fish with the tank sides and bottom, due to a higher ratio of tank volume:tank wall and bottom. Many species prefer the consistent current of a round tank to other systems. Rounded tanks are ideal for marine usage because there is less constraint on their size in such environments as even relatively large round tanks can be stable in the water.